1. Field of the Invention
This invention relates generally to infrared (IR) scanners and more specifically to multifaceted scanning wheels for such scanners.
2. Description of Related Art
Infrared scanners, to which the present invention relates, typically comprise a multifaceted scanwheel mounted for rotation on a bearing, means for aiming infrared light at the facets as the wheel rotates at high speed and for detecting the light reflected by the facets, and a drive motor and associated drive electronics for spinning the scanwheel at a constant, controlled speed. Important considerations in the design of such scanners is that the scanwheel facets be very precisely oriented relative to the axis of rotation of the wheel, and that the friction encountered by the bearing be as low as possible for minimal power consumption. When such scanners are used in military applications, they are subjected to shock loads up to 100 G and vibration and, hence, must be strong and mechanically stiff, with a resonant frequency above a certain level such as 500 Hz. They also are required to work under extremes of temperature which may range from -55.degree. to +72.degree. C. Low power consumption and minimum weight are frequently additional design requirements. The bearing is usually made of steel for strength, the scanwheel of aluminum for minimum weight. These metals have radically different coefficients of thermal expansion and of conductivity. As a result, when the scanwheel assembly is exposed to expected changes in temperature, stresses are introduced which can result in compression of the bearing, causing increased power consumption and distortion of the scanner wheel facets, degrading optical performance. Measures to isolate the bearing and facets from thermally induced stresses tend to conflict with the need for resistance to shock and vibration. This is so because stress isolation calls for increased flexibility, whereas resistance to shock and a high resonant frequency call for increased stiffness.